1. Technical Field
The present invention relates to a voltage detection apparatus, and particularly to a voltage detection apparatus for detecting a voltage of a vehicle-mounted high-voltage battery made of plural unit cells mutually connected in series.
2. Background Art
In recent years, a hybrid vehicle (hereinafter called HEV: Hybrid Electric Vehicle) traveling with a combination of an engine and an electric motor has become widespread. This HEV includes two kinds of batteries of a low-voltage battery of about 12 V for starting of the engine and a high-voltage battery for driving of the electric motor. The high-voltage battery described above obtains a high voltage by using a secondary battery such as a nickel-hydrogen battery or a lithium battery as a unit cell including at least one or more cells or secondary batteries and connecting the plural unit cells in series.
While the high-voltage battery described above repeats charge and discharge, variations occur in a voltage between both ends of each of the secondary batteries, that is, a state of charge (SOC). In the case of charge and discharge of the battery, it is necessary to prohibit charge at a point in time when the secondary battery with the highest SOC (or voltage between both ends) reaches a set upper limit SOC (or upper limit voltage value between both ends) and prohibit discharge at a point in time when the secondary battery with the lowest SOC (or voltage between both ends) reaches a set lower limit SOC (or lower limit voltage value between both ends) from the standpoint of ensuring of safety or durability of each of the secondary batteries. Therefore, when variations in the SOC occur in each of the secondary batteries, a usable capacity of the battery decreases substantially. As a result of this, in the HEV, the so-called assist and regeneration, in which battery energy is replenished with respect to gasoline at the time of ascent or energy is regenerated in the battery at the time of descent, become insufficient and actual vehicle power performance or fuel consumption is reduced. Hence, it is necessary to detect a voltage between both ends of each of the secondary batteries in order to equalize the SOC of each of the secondary batteries.
An apparatus as shown in JP-A-2003-243044 is proposed as a voltage detection apparatus for detecting a voltage between both ends of each of the secondary batteries constructing the high-voltage battery described above. The voltage detection apparatus of JP-A-2003-243044 divides the high-voltage battery into plural modules and detects a voltage between both ends of each of the secondary batteries of the inside of each of the modules by a CPU etc. arranged every module. By being constructed thus, voltages between both ends of plural secondary batteries can be simultaneously detected and also a withstand voltage of a device used for detecting the voltage between both ends can be decreased.
In the voltage detection apparatus described above, a detection error occurs between modules due to variations in accuracy of an A/D converter, a reference voltage, etc. used in detection of a voltage between both ends of each of the modules. When the detection error between the modules is large, an equalization error occurs in the case of equalizing the SOC of each of the secondary batteries, so that each of the secondary batteries could not be used efficiently. When an A/D converter or a reference voltage with high accuracy is used, this error can be eliminated, but there was a problem that the A/D converter or the reference voltage with high accuracy is required by the number of modules divided and a cost increases.
A voltage correction method described in US2006/0273802A is proposed as a method for correcting such an error. In the voltage correction method described in US2006/0273802A, an average value of measured values is calculated every each block and is set as a representative value of the block and its representative value is compared. When there is a difference between the representative values, its difference is corrected.
However, the voltage correction method described in US2006/0273802A had a problem that though the voltage detection itself is performed normally when the voltage itself of a block varies, a difference in voltage is corrected as an error, in other words, the difference is corrected as an error of a measurement system in an unknown state as to whether the voltage itself of the block varies or an error is the error of the measurement system.